Problem: Simplify the following expression: $p = \dfrac{t}{4} - \dfrac{7t}{9}$
Answer: In order to subtract expressions, they must have a common denominator. The smallest common denominator is the least common multiple of $4$ and $9$ $\lcm(4, 9) = 36$ $ p = \dfrac{9}{9} \cdot \dfrac{t}{4} - \dfrac{4}{4} \cdot \dfrac{7t}{9} $ $p = \dfrac{9t}{36} - \dfrac{28t}{36}$ $p = \dfrac{9t -28t}{36}$ $p = \dfrac{-19t}{36}$